Direct current (DC) motors are able to produce torque because a current-carrying conductor located in a magnetic field experiences a force proportional to the magnitude of the flux, the current, the length of the conductor, and the sine of the angle between the conductor and the direction of the flux. When the conductor is a fixed distance (radius) from an axis about which it can rotate, a torque is produced that is proportional to the product of the force and the radius. In a DC motor, the resultant torque is the sum of the torques produced by each conductor. Each of the conductors are known as windings, and it is important for the windings to be insulated from each other so that the current flowing through them will not short circuit from one winding to another.
DC motors should be designed so that they will not overheat during usage. If the windings on a DC motor reach a temperature at which the protective coating, or insulation on the conductors melts, then the motor may short-circuit and fail. In addition to ambient conditions, things which affect the temperature of a DC motor can include the design and size of the DC motor, the magnitude of the load which the DC motor is coupled to, or even changes to the efficiency of the DC motor over time. If the temperature of a DC motor cannot be measured while the DC motor is in use, then the DC motor must be designed or selected robust enough to handle the worst-case loads it can possibly see over the expected lifetime of the DC motor. Often, this means a relatively large DC motor must be selected. Using such a large DC motor may add significant cost to a product containing the DC motor.